Directly modulated or externally modulated laser optical transmission system with feed forward noise cancellation

Abstract

An optical transmitter for generating a modulated optical signal for transmission over a fiber optic link to a remote receiver including a laser; a modulator for directly amplitude modulating the laser with an analog RF signal to produce an optical signal including an amplitude modulated information-containing component; and a phase modulator coupled to the output of the laser for canceling the noise signals generated in the laser.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of this invention will be better understood and more fully appreciated by reference to the following detailed description when considered in conjunction with the accompanying drawings, wherein:

FIG. 1( a) is a highly simplified block diagram of an externally modulated optical transmission system as known in the prior art;

FIG. 1( b) is a highly simplified block diagram of a directly modulated optical transmission system as known in the prior art;

FIG. 2 is a highly simplified block diagram of the optical transmission system according to the present invention.

Claims

1. An optical transmitter for generating a modulated optical signal for transmission over a fiber optic link to a remote receiver comprising: a laser for producing a baseband optical signal including noise spread over the frequency spectrum;a modulator for directly amplitude modulating the laser with an analog RF signal to produce an optical signal including an amplitude modulated information-containing component and a phase modulated component; anda phase modulator coupled to the output of the laser for canceling the phase noise associated with the optical signal.

2. A transmitter as defined in claim 1, wherein the laser is a semiconductor laser and the phase modulator cancels the noise component in the laser's output signal.

3. A transmitter as defined in claim 1, wherein the phase modulator increases the SBS threshold of the received optical signal at the remote receiver.

4. A transmitter as defined in claim 1, further comprising a frequency discrimination circuit having an input connected to the output of the laser and an output coupled to a photodiode so that the phase noise in the optical signal is converted to a modulating electrical signal applied to the phase modulator such that effective phase noise cancellation occurs.

5. A transmitter as defined in claim 1, wherein the wavelength of the light output of the laser is in the 1530 to 1570 nm range.

6. A transmitter as defined in claim 1, wherein broadband analog signal input has a bandwidth greater than one octave and includes a plurality of distinct information carrying channels.

7. A transmitter as defined in claim 1, further comprising a predistortion circuit for modifying the RF signal applied to the laser to compensate for the nonlinear response of the laser affecting the signal at the remote receiver.